Although effective treatments for obsessive compulsive disorder (OCD) are increasingly available, a considerable percentage of patients fails to respond or relapses. Predictors associated with improved outcome of OCD were identified. However, information on interpersonal determinants is lacking. This study investigated the contribution of attachment style and expressed emotion to the outcome of exposure and response prevention (ERP), while accounting for previously documented intrapersonal (i.e., symptom severity and personality pathology) predictors. Using logistic regression analyses and multi-level modeling, we examined predictors of treatment completion and outcome among 118 adult OCD patients who entered ERP. We assessed outcome at post treatment, and at four and 13 months from treatment completion. OCD baseline severity and fearful attachment style emerged as the main moderators of treatment outcome. Severe and fearfully attached patients were more likely to dropout prematurely. The improvement of fearful clients was attenuated throughout treatment and follow-up compared to non-fearful clients. However, their symptom worsening at the long-term was also mitigated. Severe OCD patients had a more rapid symptom reduction during treatment and at follow-up, compared to less severe clients. The findings suggest that both baseline OCD severity and fearful attachment style play a role in the long-term outcome of ERP.